xml file gets deleted unexpectedly on a GET request

I am reading text from a xml file, and insert this text into a Label. I read the text from the xml file every 5th second, in case it has changed. This mean that I insert the text into the label every 5th second (no matter if it has changed or not). Now the problem is that my xml file gets deleted when I do a GET the 2nd time, and this results in a 500 (internal server error), since the file is no longer there.

Here is the code that creates the xml document

[WebMethod]
[ScriptMethod(UseHttpGet=true)]
public string GetMessage() {
    XmlTextReader reader = new XmlTextReader (Global.sAppPath + "/alt/importantMsg.xml");

    string message = null;

    while (reader.Read()) {
        if (reader.IsStartElement ()) {
            switch (reader.Name.ToString ()) {

            case "Message":
                message = reader.ReadString();
                break;
            }
        } 
    }       
    return message;
}

And here is the code that issues a GET

    window.setInterval(function() {
        msg;

        $.ajax({
            url: "http://ift.tt/1rvYRPu",
            type: "GET",
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            success: function(data) {
                msg = data.d;
            },
            error: function() { 
                alert("Error");
            }
        }); 

        Ext.getCmp('attentionLabel').setText(msg);
        Ext.getCmp('attentionLabel').show();

}, 5000); 

Below is the code that issues a POST, to save the message to the xml file, along with the client code

[WebMethod]
public void AddToXMLFile (string param)
{
    XmlDocument doc = new XmlDocument();
    DateTime today = DateTime.Today; 

    XmlNode rootNode = doc.CreateElement("Scandiatransplant");
    XmlNode dateNode = doc.CreateElement("Date");
    XmlNode importantMsg = doc.CreateElement("Message");

    XmlAttribute msgId = doc.CreateAttribute("id");
    msgId.Value = "msgID";

    dateNode.InnerText = today.ToString();
    importantMsg.InnerText = param;
    importantMsg.Attributes.Append(msgId);

    rootNode.AppendChild(dateNode);
    rootNode.AppendChild(importantMsg);

    doc.AppendChild(rootNode);
    doc.Save(Global.sAppPath + "/alt/importantMsg.xml");
}

And the client code

 function saveMsg(msg) {
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", "http://ift.tt/1rweS84", true); //async
    xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
    xmlhttp.send("param=" + msg);
}

Can anyone see why the xml file suddenly gets deleted, which results in an internal server error?